The present embodiments relate to wireless communication systems and, more particularly, to the transmission of Hybrid Automatic Repeat Request acknowledgments in response to multi-input multi-output (MIMO) transmissions of data and dedicated reference signals with codebook-based feedback on the Physical Downlink shared channel (PDCCH).
With Orthogonal Frequency Division Multiplexing (OFDM), multiple symbols are transmitted on multiple carriers that are spaced apart to provide orthogonality. An OFDM modulator typically takes data symbols into a serial-to-parallel converter, and the output of the serial-to-parallel converter is considered as frequency domain data symbols. The frequency domain tones at either edge of the band may be set to zero and are called guard tones. These guard tones allow the OFDM signal to fit into an appropriate spectral mask. Some of the frequency domain tones are set to values which will be known at the receiver. Among these are Cell-specific Reference signals (CRS), Channel State Information Reference Signals (CSI-RS) and Dedicated or Demodulating Reference Signals (DMRS). These reference signals are useful for channel estimation at the receiver for data demodulation and also to support link adaptation at the transmitter. In a multi-input multi-output (MIMO) communication systems with multiple transmit/receive antennas, the data transmission is performed via precoding. Here, precoding refers to a linear (matrix) transformation of a L-stream data into P-stream where L denotes the number of layers (also termed the transmission rank) and P denotes the number of transmit antennas. With the use of dedicated user-specific DMRS, a transmitter (base station, also termed eNodeB or eNB) can perform any precoding operation which is transparent to a user equipment (UE) which acts as a receiver. At the same time, it is beneficial for the base station to obtain a recommendation on the choice of precoding matrix from the user equipment. This is particularly the case for frequency-division duplexing (FDD) where the uplink and downlink channels occupy different parts of the frequency bands, i.e. the uplink and downlink are not reciprocal. Hence, a codebook-based feedback from the UE to the eNodeB is preferred. To enable a codebook-based feedback, a precoding codebook needs to be designed. UE measures the downlink MIMO channel and feeds back the channel by using the feedback codebook. Specifically, UE reports a precoding matrix indicator (PMI) corresponding to a recommended precoding matrix from the feedback codebook, as well as channel quality indicators (CQI) which reflects the receive signal quality when the recommended PMI is used for MIMO precoding.
The Rel. 8 Long-Term Evolution (LTE) specification includes a codebook for 2-antenna transmissions and a codebook for 4-antenna transmissions. While those codebooks are designed efficiently, they do not support transmissions with 8 antennas. Moreover, it is possible to further improve the performance of 4-antenna transmissions under different scenarios such as dual-polarized antenna arrays. To address these issues, an 8-Tx codebook was adopted in LTE Rel. 10 for the purpose of channel feedback for an 8-antenna system.
While the preceding approaches provide steady improvements in wireless communications, the present inventors recognize that still further improvements in downlink (DL) spectral efficiency are possible. Accordingly, the preferred embodiments described below are directed toward these problems as well as improving upon the prior art.